The annual reproductive cycle of the Korean endemic slender catfish, Silurus microdorsalis, was examined histologically regarding water temperature and day length of habitat, gonadosomatic index (GSI), and development characteristics of female and male gonads. The maximum GSI value was found in May, 1.23±0.33 and 11.77±3.23 for male and female respectively (habitat water temperature 21.5℃/13.59hr day length). On the other hand, the minimal level was 0.63±0.10 in July (26.5℃/14.17) for male and 1.36±0.08 in October (20℃/11.2hr) for female. We compared and calculated the stages of testis and ovary development process in order to determine the germ cell development characteristics and the reproductive cycle. According to results, we classified the annual reproductive cycle of the slender catfish into five stages: Growing phase (December- February), Mature phase (March-April), Ripe and spawning phase / Releasing phase in male (May-June), Degenerative phase (July-August), and Resting phase (September-November).